(1) Field of the Invention
The present invention relates to an aircraft hydraulic system comprising at least one servo-control, and also to a rotor and to an aircraft fitted with the hydraulic system.
(2) Description of Related Art
Conventionally, an aircraft has control members, such as the blades of a rotor providing a rotorcraft with lift, or indeed the control surfaces of an airplane, for example.
Using flight controls, the pilot thus operates the control members of the aircraft. Nevertheless, the forces that need to be delivered in order to move such control members are sometimes very large.
Consequently, the linkage connecting a flight control to a control member is often provided with a hydraulic system including a servo-control enabling the pilot to control the aircraft without difficulty and accurately.
More particularly, a rotorcraft has a main rotor providing the rotorcraft with at least some of its lift and possibly also with propulsion. In order to direct the rotorcraft, the pilot can in particular modify the pitch of the blades of the main rotor.
Consequently, the rotorcraft has a “set of swashplates” comprising a non-rotary bottom swashplate and a rotary top swashplate. Below, this set is indeed referred to as a set of swashplates. In contrast, it should be understood that the person skilled in the art sometimes refers to this set of swashplates more simply as the “swashplates”. The non-rotary bottom swashplate is connected to the pilot's flight controls, generally via at least three distinct control systems, while the rotary top swashplate is connected to each of the blades via a respective rod. The set of swashplates thus slides along the mast of the main rotor in order to control the collective pitch of the blades of the main rotor. Furthermore, this set of swashplates can oscillate about a ball joint slidably mounted on the mast in order to control the cyclic pitch of the blades.
The movements in oscillation and in translation of the set of swashplates thus serve to vary the pitch of the blades and thereby enable a pilot specifically to direct a rotorcraft.
The pilot sometimes controls the set of swashplates via mechanical controls. Nevertheless, the forces a pilot needs to exert in order to move the set of swashplates are very large, in particular when the weight of the rotorcraft is also large.
Consequently, a servo-control is arranged between an upstream portion and a downstream portion of each controlled-linkage system. The pilot then acts on the servo-controls without applying large amounts of force via the upstream portion, and the servo-controls copy an order from the pilot so as to act on the downstream portion of the linkage system.
Likewise, a helicopter has a tail rotor and the pitch of its blades can be modified by means of a servo-control in order to control yaw movements of the aircraft.
Naturally, the same applies for example to the ailerons or the flaps of airplanes when they are controlled via servo-controls.
Usually, a servo-control includes an actuator having at least an outer cylinder and a drive rod. The drive rod then has a control piston. Each control piston can move in translation inside a corresponding cylinder. Thus, each control piston defines a retraction chamber and an extension chamber inside the corresponding outer cylinder.
Furthermore, the servo-control has a hydraulic valve for feeding fluid to the retraction chamber or to the extension chamber depending on the received order.
A piloting order from a pilot is thus transmitted to the hydraulic valve, and it is the hydraulic valve that feeds fluid to the appropriate hydraulic chambers. As a function of the orders that are given, the hydraulic valve thus feeds hydraulic fluid to each retraction chamber or to each extension chamber, and consequently causes the servo-control to retract or to extent.
It should be understood that in the text below, the term “retraction chamber” is used to refer to a chamber causing the servo-control to retract when said chamber is filled with a fluid. Conversely, the term “extension chamber” is used to designate a chamber that causes the servo-control to extend when said chamber is filled with a fluid.
The servo-control may also comprise a servo-control device, possibly incorporated in the hydraulic valve.
The servo-control then has multiple dynamic gaskets arranged on its moving parts.
A first dynamic gasket may be arranged on each control piston. Such a dynamic gasket serves to prevent undesired passage of fluid from the retraction chamber to the extension chamber of a cylinder.
A first dynamic gasket that leaks can lead to a degradation in the performance of the servo-control.
A second dynamic gasket is also arranged between the drive rod and the corresponding cylinder of the servo-control.
A second dynamic gasket that leaks allows fluid to escape to the outside of the servo-control, and consequently onto other pieces of equipment. The leak can be detected by visual inspection and leads to the servo-control being repaired.
One kind of failure for a servo-control thus relates to at least one of its gaskets wearing so as to give rise to a leak.
A servo-control therefore sometimes has a detector device for monitoring the pressure of the fluid of the hydraulic circuit in order to detect a leak that is large. If a large leak is detected, the user is warned and can then undertake measures recommended by the manufacturer. On an aircraft, it is common practice to use servo-controls having at least two cylinders, in particular for safety purposes. Thus, if one cylinder becomes inoperative as a result of an accidental leak, the servo-control can remain functional.
It is also possible to imagine being confronted with so-called “dormant” failures, particularly on a servo-control having at least two cylinders.
Such a dormant failure can appear in the event of a first dynamic gasket leaking Such an intermediate leak may for example allow excessive hydraulic fluid to pass between the retraction chamber and the corresponding extension chamber of a cylinder.
Under such conditions, a pilot runs the risk of not physically perceiving the malfunction of the servo-control insofar as at least one other cylinder of the servo-control remains operational.
In order to detect such dormant failures, a manufacturer may provide for complete maintenance operations to be performed at regular time intervals. Such maintenance operations represent a cost that is not negligible.
In summary, a servo-control is conventionally provided with dynamic gaskets that run the risk of giving rise to excessive leaks in the event of deterioration.
Unfortunately, using a servo-control leads to its dynamic gaskets being heated by friction. Depending on the frequency with which the servo-control is operated, such heating degrades dynamic gaskets and leads to their destruction.
The presence of dynamic gaskets thus constitutes limit on the performance of a servo-control.
Documents GB 2 026 406, U.S. Pat. No. 2,861,640, FR 2 313 603, U.S. Pat. Nos. 4,105,365, 5,387,083, and 3,007,530 are all known.